The simple pedometer has been given a makeover. Fitbit, a startup based in San Francisco, has built a small, unobtrusive sensor that tracks a person’s movement 24 hours a day to produce a record of her steps taken, her calories burned, and even the quality of her sleep. Data is wirelessly uploaded to the Web so that users can monitor their activity and compare it with that of their friends.

James Park, cofounder of Fitbit, says that one of the main goals was to make the sensor so small that it will go unnoticed no matter what a person is wearing. The device can be put in a pocket, attached discreetly to a bra, or slipped into a special wristband during sleep. It is meant to be worn 24-7, and each device can run for 10 days on a single battery charge. Park demonstrated the Fitbit device in San Francisco on Tuesday at the Techcrunch50 conference, a popular launch pad for new technology companies.

At the conference, the gadget impressed a panel of judges that included Tim O’Reilly of O’Reilly Media. He says that Fitbit is tapping into an important field of wearable sensors and personal health monitoring: “It’s completely on trend in terms of this idea of sensors driving the next generation of interesting applications.”

Evan Williams, cofounder of both Blogger and Twitter, adds that, while the concept is simple, it appears to be well executed. “The design of the product and website is strong,” he says.

For years, runners and walkers have used pedometers to track their exercise routines, but these devices can be relatively bulky and provide only a limited amount of information. Some newer pedometers connect to computers so that people can track their exercising in detail, but the process is often cumbersome. For example, Nike offers a sensor device for runners called Nike+iPod that is built into specialized shoes. The shoes transmit data to an iPod that, in turn, uploads the data to the Web when the iPod is synced with a computer.

During his demonstration at the conference, Park walked onstage for 17 steps, past the Fitbit base station. He then refreshed his information on the Fitbit website to show that his total steps for the day had already been updated. Importantly, Park noted that Fitbit has built-in technology to distinguish between the motion of a car and a person walking or running.

Another feature that Park believes sets Fitbit apart is the way that its Web service automatically converts steps taken into burned calories and lets people compare their activity with that of other Fitbit users. “I can get a real-time activity feed from my friends,” says Park. The site can also log meals and create calorie budgets to tie into a diet regime.

At night, the sensor fits into a wristband, and its accelerometer tracks tiny tremors in the wrist that correlate to different stages of sleep. When sleep-related data is uploaded to the Web, it is used to create a graph showing the amount and quality of sleep achieved each night.

Park says that Fitbit isn’t meant to replace a sports pedometer; rather, it’s meant to give people a better sense of their daily activity and act as a dieting aid. The sensor will be available by December or January, Park says, and will retail for $99; use of the Fitbit website will be free.